Chronic lower back pain caused by degenerative disc disease is one of the leading causes of disability in adults. Intervertebral disc degeneration can occur as part of the normal aging process in which the nucleus of the disc dehydrates, reducing the shock absorbing capability of the disc. Patients who fail to obtain adequate pain relief from non-surgical treatment (e.g., rest, pain medication, physical therapy, exercise, epidural steroid injections, chiropractic manipulation, ultrasound, massage, orthotics, etc.) may require spinal surgery to alleviate discogenic pain and disability.
One method of treating degenerative disc disease is spinal fusion or arthrodesis surgery in which the affected vertebrae are fused together using a bone graft. During spinal fusion, a perforated titanium cage may be surgically implanted within the space between two adjacent vertebrae after the pain-generating intervertebral disc is removed. The implanted spinal fusion cage must be appropriately sized to restore the normal disc height at the affected vertebral segment. The fusion cage is packed with bone graft, which grows through the perforated walls of the cage and eventually forms a solid bond or fusion with the adjacent vertebrae to prevent motion in the affected vertebral segment and reduces chronic discogenic pain.
Another approach for treating degenerative disc disease is total disc replacement (“TDR”), which seeks to alleviate discogenic pain, while maintaining or minimizing the loss of motion in the affected vertebral segment. During TDR, the pain-generating intervertebral disc is removed and a metallic artificial disc implant that allows motion, such as the ProDisc™-L manufactured by Synthes Spine of West Chester, Pa. and Charité® manufactured by DePuy Spine, Inc. of Raynham, Mass., is inserted into the space between the adjacent vertebrae. The TDR implant must be appropriately sized to restore the normal disc height at the affected vertebral segment, thereby reducing chronic discogenic pain, while maintaining or minimizing loss of range of motion in the affected vertebral segment.
While spinal fusion and TDR have been successfully used to treat degenerative disc disease, it is estimated that between 3-8% of these surgeries must be surgically revised. Some of these revisions are believed to be due to the improper selection, sizing and placement of the artificial disc or fusion cage, which is based predominantly on the judgment of the surgeon at the time of the procedure.
For example, surgeons today must rely on their experience and “feel” when using intervertebral spreaders or distractors to spread the affected intervertebral segment during TDR or fusion surgery. Both the artificial disc and fusion cage must fit in a “snug” intervertebral space that has appropriate ligament tension provided by the anterior longitudinal ligament, posterior longitudinal ligament (if it has not been removed), ligamentum flavum, facet capsular ligament (in the case of TDR where the facet is not removed), and the inter-transverse and inter-spinous ligaments. Too much distraction by the surgeon performing a TDR procedure will result in the placement of an implant that is too large for the intervertebral segment, reducing the ideal range of motion that the implant can provide. In contrast, insufficient ligament tension will not produce enough force on the endplates of the arthroplasty, resulting in loosening and ultimately subsidence (i.e., migration of the implant from its optimal position). Similarly, since fusion cages or devices should be placed in a stretched intervertebral segment for better fusion, insufficient ligament tension may result in an unsatisfactory fusion and/or undesired range of motion of the affected vertebral segment.
In addition, anterior and posterior pre- and post-distraction disc heights are important parameters for a surgeon to consider when placing an intervertebral disc arthroplasty. It is believed that there is an optimal window of anterior and posterior disc height that allows the optimal range of motion by the arthroplasty. Currently, surgeons must either “eyeball” anterior and posterior disc heights before and after distraction on a fluoroscope, and/or use so called “Yo-Yo” or trial devices to measure the size of the intervertebral space by forcing different Yo-Yo's into the space between adjacent vertebras until the correct sized Yo-Yo is placed. However, these Yo-Yo devices do not allow for ligament tension measurement and/or feel by the surgeon. Conventional intervertebral spreaders do not measure anterior and posterior disc heights.
Yet another parameter for a surgeon to consider when placing an intervertebral disc arthroplasty is the depth of the patient's vertebral endplate. New arthroplasties should cover the entire vertebral endplate outer-rim because this is where the strongest bone lies. However, neither conventional distractors/spreaders nor fluoroscopes provide a surgeon with the precise depth of the patient's vertebral endplate to select the correct size arthroplasty or fusion device that will cover the maximum amount of outer rim.
Intervertebral angle is also an important parameter that surgeons must estimate based on their experience using fluoroscopes and x-rays. Estimating the correct “lordotic” or “kyphotic” angle is important because it allows for the correct spinal segment alignment in the case of fusion devices, or for the maximum range of motion in the case of total disc arthroplasties. Conventional spreaders (distractors) or Yo-Yo devices do not provide for a precise determination of the intervertebral angle.
It is, therefore, desirable to reduce post-operative complications of spinal fusion and TDR arising from improper selection, sizing and placement of the artificial disc or fusion cage.